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Boilers

Operation and Maintenance Tips

A systems approach is the best way to save energy. This means looking at the boiler, the steam distribution system and the end uses together. Learn more about saving energy in steam and hot water distribution systems.

Following are some common energy-saving tactics.

Control excess combustion air. Controlling excess air is the most important tool for optimizing boiler efficiency. Too little air results in incomplete combustion, while too much air wastes energy, as the excess air is heated to the stack temperature.

Reducing the excess air nearly always yields a greater increase in efficiency. This results from a reduction in the flue gas temperature due to reduced mass flow and consequent improved heat transfer through the system. Stack temperature and flue gas oxygen (or carbon dioxide) concentrations are primary indicators of combustion efficiency.

A number of controls are available for monitoring and optimizing the air-fuel mixture. These range from simple, low-cost on-off control to more expensive automatic oxygen trim control. Burner size determines which is the right control. The burner should be adjusted only by qualified personnel, so work with a supplier to correct the air-fuel mixture.

Train personnel. Have only well-trained, qualified personnel run, adjust, inspect and maintain boiler systems.

Keep the boiler clean. The fireside of the boiler tubes can accumulate deposits from burning fuel. This fouling can dramatically reduce heat transfer. Boilers that use solid fuels tend to foul much more than liquid- and gas-fuelled boilers. No. 6 (resid, heavy) oil has a greater fouling tendency than No. 2 oil. Natural gas boilers have a very low fouling tendency.

The waterside of the boiler tubes can become covered with a mineral deposit, or “scale”. Scale causes the tube's temperature to rise, raising the flue gas temperature and reducing the efficiency. Scale buildup can be tested with an automatic sensor while the boiler is running and can be treated chemically.

Boiler water should be tested daily in small low-pressure boilers and hourly in large high-pressure boilers. A gradual rise in flue gas temperature usually indicates that a deposit is accumulating on either the fireside or the waterside. If flue gas temperatures are too high, clean the system and adjust the water chemistry and the air-fuel mixture.

Large boilers often have soot blowers to clean fireside tube surfaces while the boiler is operating. Soot blowing can consume large amounts of energy, so it must be done carefully. Smaller boilers should be opened regularly for inspection and cleaning.

Minimize boiler short-cycling losses. When a boiler is too big, boiler short-cycling losses may occur. An oversized boiler will turn on and off more often than a boiler that has been properly matched to the demand. Every time the boiler turns on, extra energy is required to heat it back up to steady-state. A number of staged (or sequenced) smaller boilers use an automatic controller to lower costs by efficiently shifting the load between boilers. Piping and controlling the multiple boilers correctly is necessary for saving energy.

Minimize wasted blowdown water. Boiler water must be blown down periodically to prevent scale from forming on boiler tubes. This process can be wasteful if too much blowdown water is used. Automatic blowdown controls measure and respond to boiler water conductivity and acidity to ensure that only the right amount of blowdown water is used.

For further savings, a blowdown waste heat recovery system that preheats boiler feedwater can improve the system's efficiency by approximately 1 percent.

Use outdoor reset. Outdoor reset is used for hot-water building heating. In older systems, hot water arrives in the distribution system at the highest temperature the boiler provides. On-off controls regulate building temperature. Outdoor reset varies the temperature of the water in the distribution system in response to outdoor temperatures. When it is cold outside, the water temperature rises to match the heat loss from the building. When it is warm outside, there is less heat loss from the building, so the distributed water is cooler. Fuel consumption is reduced.

Conservative estimates put savings at 10 to 15 percent. This can also be an effective way of increasing latent heat recovery for high-efficiency condensing boilers.

Improve boiler insulation. A simple and cost-effective way of reducing heat loss through radiation and convection is by adding insulation directly to the outer walls of the boiler. Removable insulation pads will reduce losses and will not interfere with maintenance.

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